The present invention relates to a light transmitting apparatus. More particularly, the present invention relates to a light transmitting circuit in a wavelength multiplexing communication system.
A light wavelength multiplexing transmitting system was recently developed, and research to put the system to practical use is progressing. This system uses a light wavelength multiplexer to multiplex and transmit more than two light signals.
In the light wavelength multiplexing transmission system, it is necessary that two adjacent wavelengths .lambda.1 and .lambda.2 in more than two light signals are separated from each other. If the two adjacent wavelengths .lambda.1 and .lambda.2 coincide with each other or are extremely close to each other, it is not possible to separate the two signals, and this situation affects the signal transmitting and receiving operation.
It is necessary to control each transmission wavelength to a set value. The oscillation wavelength of a semi-conductor laser device varies on the basis of the temperature of the device, which is approximately the same as ambient temperature. Therefore, the conventional light transmitting circuit is provided with a LD (laser diode) driver for receiving a data signal to be transmitted and outputting a driving current to drive a semi-conductor laser device, a LD module for (a) outputting a light signal based on the driving current input from this LD driver and also (b) receiving an input of a temperature control signal to thereby change a temperature and (c) outputting the temperature as a temperature monitor signal; and a temperature control circuit for comparing a temperature set signal input from an external portion with the temperature monitor signal input from the LD module thereby to output the temperature control signal.
A light transmitting circuit which has different oscillation wavelengths is obtained by setting the temperature set signal in the temperature control circuit such that the oscillation wavelength is different for each light transmitting circuit.
This conventional light transmitting circuit usually sends out a light signal having a wavelength based on the temperature set signal. However, the temperature within the LD module is typically different from the set temperature right after a set-up operation such as turning on a power supply. This causes a problem in that a wavelength different from the wave length based on the temperature set signal is sent out. For example, when a light transmitting circuit A was already set up, if another light transmitting circuit B is set up later, there is a possibility that the other transmitting apparatus B may temporarily send out the signal having the wavelength of the light transmitting circuit A, until the wavelength of the light transmitting circuit B becomes stable.